WO2013056968A1

WO2013056968A1 - Sputtering target and use thereof

Info

Publication number: WO2013056968A1
Application number: PCT/EP2012/069278
Authority: WO
Inventors: Materials Technology Gmbh & Co. Kg Heraeus; zur Förderung der angewandten Forschung e.V. Fraunhofer-Gesellschaft; Andreas Herzog; Markus Schultheis; Sabine Schneider-Betz; Martin Schlott; Wilma Dewald
Original assignee: Heraeus Materials Tech Gmbh; Fraunhofer-Gesellschaft Zur Foerderung Der Angewan
Priority date: 2011-10-19
Filing date: 2012-09-28
Publication date: 2013-04-25
Also published as: KR20140063814A; TW201326431A; EP2584062A1; JP2015502452A; CN104011255A

Abstract

The present invention relates to a sputtering target made of a material that contains at least two phases or components. One phase is formed by a metal oxide forming the matrix, in which the elemental metal or metal alloy is embedded. The sputtering target is characterised through its low resistance and high density. It is well-suited for producing layers of low reflectivity and high absorptivity.

Description

Sputtering target and use thereof

The invention relates to a sputtering target made of a material that contains at least two phases or components. The invention relates, in particular, to a sputtering target that contains oxide and elemental metal. The invention also relates to the use of said sputtering targets.

Liquid crystal displays (LCDs) and absorber layers for applications of solar thermal energy have been indispensable elements of the electronics industry for a long time. They are used in many areas of information technologies and there is a steady demand for image displays of larger size and higher resolution. In order to improve the contrast of displays, light-absorbing thin layers of CrO_x/Cr have been developed since the 1980s. Pertinent examples are disclosed in US patent no. 5,976,639.

It is customary to apply the light-absorbing thin layers through deposition by sputtering ("sputtering"). Sputtering involves atoms or compounds being detached from a solid body, the so-called sputtering target, by bombardment with energy-rich ions (usually noble gas ions) and then entering into the gas phase. The atoms or molecules in the gas phase are ultimately deposited by condensation on a substrate in the vicinity of the sputtering target where they form a layer.

Due to the toxicity of chromium, there is an urgent need for these layers to be free of chromium. For this reason, there is an ongoing search for improved semiconductor materials. Other oxides, in particular, have proven to be promising alternatives.

For example US 6,387,576 B2 discloses a light-absorbing layer, a so-called "black matrix", that contains SiO as dielectric material and at least one further metal, which can be iron, cobalt, vanadium or titanium. In this context, the SiO content decreases along the incidence direction of the light to the display, whereas the metal content increases. The layers thus formed must have a layer thickness of at least 0.2 μιτι to meet the optical requirements.

The present invention is therefore based on the object to provide a sputtering target that is well-suited for producing light-absorbing, high-resistance, and electrically insulating layers of sufficiently high reflectivity and sufficiently high absorptivity even at layer thicknesses of less than 0.2 μιτι.

Said object is met by a sputtering target having the features of claim 1 . This is a sputtering target made of a material that contains at least two phases or components, whereby a reduced metal oxide forms the matrix and elemental metal or a metal alloy is embedded in said oxide matrix. The object is also met through the use of said sputtering targets for producing light-absorbing high-resistance layers according to claim 1 1 . In the scope of the invention, a reduced metal oxide shall be understood to mean a metal oxide that comprises less oxygen than the stoichiometric composition. In the scope of the invention, "metal oxide forms the matrix" shall be understood to mean that the metal oxide that is present accounts for more than 50 vol.% to 98 vol.%, and preferably accounts for more than 55 vol.% or more. The metal or the metal alloy is present in the matrix in finely dispersed form. It accounts for 2 to less than 50 vol.%, preferably for 2 vol.% up to 45 vol.% or less.

In a particularly preferred embodiment, the target according to the invention contains, as matrix, metal oxide accounting for 90 vol.% - 98 vol.% and elemental metal or metal alloy accounting for 2 vol.% - 20 vol.%.

In another particularly preferred embodiment, the target according to the invention contains, as matrix, metal oxide accounting for 55 vol.% - 70 vol.% and elemental metal or metal alloy accounting for 30 vol.% - 45 vol.%. The metal oxide can be any reduced metal oxide. Preferably, said oxide is an oxide from groups 4-6 of the period system of the elements, preferably selected from the group consisting of any oxide modification of titanium oxide, any oxide modification of niobium oxide, any oxide modification of vanadium oxide, any oxide modification of molybdenum oxide, any oxide modification of tantalum oxide, any oxide modification of tungsten oxide or a mixture thereof. Reduced niobium oxide, in particular N₂O5_-x, is particularly preferred.

The metal embedded in the matrix preferably is Ag or Al with Ag being particularly preferred. Metal alloys can be embedded in the matrix just as well with metal alloys containing Ag or Al being preferred.

Normally, two-component materials consisting of an oxide and a metal, in which the volume fraction of the metal is no more than 20 %, are non-conductive and can therefore not be used as DC sputtering target. Only above a volume fraction of approx. 15 % - 30 %, depending on the morphology and quantity of the conductive phase, a percolation network is formed which renders the target conductive.

However, according to the invention, a reduced metal oxide is used, i.e. a metal oxide comprising less oxygen as compared to the stoichiometric composition. This renders the sputtering target conductive even if no percolating metal network is formed.

Preferably, its specific electrical resistance is 1 .5 Qcm or less, more preferably 0.35 Qcm or less, in particular 0.1 Qcm or less, and particularly preferably 0.05 Qcm or less. The lower limit of a specific electrical resistance can be specified to be 10^"5 Qcm.

The sputtering target can consist of the mixture specified above. In contrast, said mixture can just as well be arranged on a carrier. The carrier can, for example, be made of stainless steel. Said carrier usually works as cathode in sputter deposition. According to a preferred embodiment, said cathode is a planar cathode or a tubular cathode. According to a particularly preferred embodiment, the cathode is a tubular cathode. If applicable, the sputtering target can comprise further components aside from the carrier and the mixture, in particular further layers that can be arranged, for example, between the carrier and the mixture. According to a preferred embodiment of the invention, the density of the sputtering target is > 85 %. Preferably, the density of the sputtering target is > 95 %. The density [%] is defined to be the ratio of the apparent density [g/cm³] and the theoretical density [g/cm³]. A method for producing the sputtering target according to the invention comprises that a mixture of the two components is produced in a first step in such manner that the metal component(s) is/are finely dispersed such that preferably no or only very small agglomerates are present, and the particles have a mean size of no more than 25 μιτι, but also to approximately 50 nm, as determined through laser diffraction. Said powder mixture is then used in known manufacturing methods, such as sintering, in an inert or reducing atmosphere to produce a planar or tube-shaped sputtering target. Hot- pressing in a non-oxidising atmosphere according to JP-A-07-233469 or a reducing plasma spraying method according to US 6,193,856 B1 are used analogously as manufacturing methods for such sputtering targets with a sub-stoichiometric oxygen content.

Mixing methods for producing powder mixtures for composites are already known.

Accordingly, a composite can be produced, for example, through mixing a sub- stoichiometric metal oxide M¹O_n-x and a metal M² on the particle level of typically 0.1 - 100 μιτι through mechanical mixing in a mixing apparatus, e.g. a mixer made by Eirich, or through spray agglomeration. Herein, n is the oxidation stage of M¹.

In order to prevent potential segregation, it can be advantageous to add binding agent and/or water. Suitable binding agents are known to the person skilled in the art.

Common binding agents include polyvinylalcohol, polyacrylates, cellulose, etc.. The components of the binding agent can serve as carbon source for (further) reduction of the metal oxide in the further manufacturing process.

The mixture can just as well contain carbon or the corresponding metal hydride. Both substances are preferably present having a particle size of 0.05 - 1 *10² μιτι. The presence of finely distributed carbon or metal hydride allows to generate a partial reduction of the metal oxide in the material. Reaction gases need to be drawn off in controlled manner during the process. Shaping of the resulting powder mixture is advantageously effected through uniaxial pressing or CIP (cold isostatic pressing). Typical pressures used for pressing are in the range of 50-200 MPa. In the subsequent step of the process, a sintering process to attain the target density is carried out under inert conditions. In this context, the sintering temperature must be matched to the melting temperature of the metal (M) or metal alloy, i.e. must be below the melting temperature of the metal or metal alloy. In the exemplary case of Ag Nb₂O₅, this means to stay below 960°C. The possible application of pressure (gas) allows the sintering activity and thus the attainable sintering density to be increased. This generates a M¹O_n-x/M² composite material that contains largely isolated metal phases. The conductivity (thermal, electrical) is provided through the reduced oxide phase.

Hot-pressing is an alternative compaction method for the mixed powders according to the description provided above. The metal oxide can be (further) reduced in direct contact to carbon in parallel. Shaping and sintering proceed in a process step at pressures of 10-40 MPa. In this context also, the sintering temperature must be matched to the melting temperature of the metal (M) or metal alloy.

The HIP (hot isostatic pressing) procedure is another alternative compaction method for the mixed powders described above. The metal oxide can be reduced by means of the methods described above. The compaction pressure is 10 - 100 MPa and the pressure profile must be matched to the progress of the sintering process. In this context, the sintering temperature must be matched to the melting temperature of the metal (M) or metal alloy. Plasma spraying is another option for preparing the composite material. In this context, a powder mixture produced as described above of metal oxide and metal or metal alloy is reduced in the suitably adjusted plasma flame and applied onto a carrier. Particle sizes of the metal or metal alloy, metal oxide, and reduced metal oxide of 10 - 100 m have proven to be advantageous.

Surprisingly, said sputtering targets can be used to provide high-resistance, light- absorbing layers that are suitable for application in liquid crystal displays.

Even at layer thicknesses as small as up to 150 nm, said layers show an absorptivity of approximately 100 % and a reflectivity of less than 15 %.

The invention also relates to the use of said sputtering target for producing a high- resistance, light-absorbing system of layers using said sputtering target. It preferably relates to the use of said sputtering targets for producing a high-resistance, light-absorbing system of layers, in which the overall thickness of all light-absorbing layers is from 30 to 150 nm. A method is particularly preferred, in which two different targets are used consecutively for producing a system of layers, whereby the first sputtering target contains niobium oxide at a volume fraction of 80 to 98 % and silver at a volume fraction of 2 to 20 %, and the second sputtering target contains niobium oxide at a volume fraction of 55 % to 65 % and silver at a volume fraction of 35 to 45 %.

The following examples are provided for the purpose of illustrating the invention and are not to be construed as to be limiting it: Example 1 :

A powder mixture of 94 vol.% ND2O 99 and 6 vol.% Ag having a mean grain size of 25 μιτι was mixed thoroughly for 1 h in a tubular mixer resulting in a fine and mono- disperse distribution of the Ag particles in Nb2O₄.99. Subsequently, said mixture was first pressed by means of cold isostatic pressing into a circular blank with a diameter of 75 mm and a height of 15 mm. Then, the circular blank was compacted through hot pressing at 940°C and 20 MPa to more than 85% of its theoretical density. The structure consisted of a Nb2O₄.99 matrix, into which individual fully deagglomerated Ag particles having a mean grain size of 25 μιτι were embedded.

A second sputtering target containing 60 vol.% Nb2O₄.99 and 40 vol.% Ag was produced in analogous manner.

Said sputtering targets were used to apply a two-layer thin film sample to a glass substrate (having a diameter of 50 mm and a thickness of 1 .0 mm). Initially, a layer containing 6 vol .% silver and having a thickness of 38 nm and then a layer containing 40 vol.% silver and having a thickness of 78 nm were applied to the glass.

The thin layer system thus deposited was subjected to measurements of the spectral reflectivity and spectral transmittance at wavelengths ranging from 400 to 800 nm using a V-570DS UV-VIS-NIR spectrometer made by JASCO. The absorptivity is calculated from the measured reflectivity and the measured transmittance according to the following equation: Absorptivity = 100% - (reflectivity + transmittance)

At 580 nm, the reflectivity is 5 % and the transmittance is 0.5 %. Accordingly, the absorptivity is determined to be 99.5 %.

Claims

Patent claims

1 . Sputtering target made of a material that contains at least two phases or components, characterised in that

a reducing metal oxide forms the matrix and elemental metal or a metal alloy is embedded in the oxide matrix.

Sputtering target according to claim 1 , characterised in that metal oxide accounts for 55 vol.% to 98 vol.%, and metal or metal alloy accounts for 2 vol.% to 45 vol.%.

Sputtering target according to claim 2, characterised in that metal oxide accounts for 90 vol.% to 98 vol.%, and metal or metal alloy accounts for 2 vol.% to 10 vol.%.

Sputtering target according to claim 2, characterised in that metal oxide accounts for 55 vol.% to 70 vol.%, and metal or metal alloy accounts for 30 vol.% - 45 vol.%.

5. Sputtering target according to any one of the claims 1 to 4, characterised in that the specific electrical resistance is 1 .5 Qcm or less.

6. Sputtering target according to any one of the claims 1 to 5, producible through sintering under inert, reducing conditions.

7. Sputtering target according to any one of the preceding claims, characterised in that the metal oxide is selected from oxides from groups 4-6 of the period system of the elements, and the metal is selected from silver and aluminium.

8. Sputtering target according to claim 7, characterised in that the metal oxide is niobium

oxide and the metal is silver.

9. Sputtering target according to any one of the preceding claims, characterised in that its density is > 85 % of the theoretical density.

10. Sputtering target according to claim 9, characterised in that its density is > 95 %.

1 1 . Use of sputtering targets according to any one of the preceding claims in a method for producing a high-resistance, light-absorbing system of layers.

12. Use according to claim 1 1 for producing a high-resistance, light-absorbing system of layers, in which the overall thickness of all light-absorbing layers is from 30 to 150 nm.

13. Use according to claim 1 1 or 12, characterised in that

two different targets are used consecutively for producing a system of layers, whereby the first sputtering target contains niobium oxide at a volume fraction of 90 to 98 % and silver at a volume fraction of 2 to 10 %, and the second sputtering target contains niobium oxide at a volume fraction of 55 % to 70 % and silver at a volume fraction of 30 to 45 %.